A telecommunications network is typically made up of a plurality of nodes which are interconnected. Routes are created through the network by choosing a set of links between the nodes so that a route can enter the network at an ingress node, traverse the network by hopping between nodes via the links and exit the network at an egress node. Typically each node has knowledge of the routes traversing that node.
Periodically, it is necessary for nodes to be restarted. Prior to the restart, it is likely that one or more route will traverse the node and thus after a restart it is necessary for the node to recover the state information about routes traversing the node in order to successfully allow the one or more routes to be resurrected after the restart. This recovery of state is typically termed “realignment” of the node.
In the prior art, two approaches typically have been used to realign nodes following a restart. In the first approach, nodes carry out periodic backups of their state to some form of non-volatile memory and are then able to restore that state following a restart. In a traditional circuit-based domain this approach is called “hard state”. In such a network (typically a time division multiplex (TDM) or dense wavelength division multiplex (DWDM) network) the circuit paths are generally quite static and do not vary over short periods of time. Thus periodic snapshots taken in the form of backups, generally work quite well because the routes in the network are unlikely to have changed in any significant way between backups. However, if significant changes to the network have been made, a very large manual effort is required to reconcile the local circuit status with the actual network status following a restart.
In other types of networks such as packet switched networks, for example Internet protocol (IP) or multi-protocol label switched (MPLS) networks, the routes change much more quickly than in traditional circuit-based networks. Accordingly a second approach to node restart has been made for this type of network, which involves a restarted node talking to neighbouring nodes in the network, i.e. nodes which are interconnected with the restarted node to gain state information about the routes in place before the restart. In the context of such networks this type of restart is known as a “graceful restart” and is described in Internet Request for Comments (RFC) 5063. This prior art graceful restart procedure assumes that only one node restarts at a time. However, if multiple restarts occur, the information available from neighbouring nodes as postulated in RFC 5063, is likely to be incomplete and the resurrection of previous routes will fail. Furthermore, if the network has fast changing routes, a backup and restore method will also fail because route changes since the last backup will often be too great and thus the restarted node will have an unacceptably outdated restarted state.
Thus there is a need for recovery of node state information to be possible in the event of multiple node restarts in a network having relatively rapid changes in routing.
It is an object of the present invention to overcome at least some of the problems of the prior art node restart approaches described above.